A fallow period on its own, or in sequence with a number of crops, can be highly effective in reducing weed seed numbers in the soil seedbank. Fallows can be initiated and maintained using cultivation. It is important that stubble cover be maintained for as long as possible to protect the soil surface during the fallow period. On mixed farms properly managed grazing can be useful in suppressing weeds, particularly the root development of weeds.
Fallows are defined as the period between two crops, or between a crop and a defined pasture phase, where the objective is to store and conserve soil moisture and nitrogen for the next crop. The term ‘fallow’ has different meanings. There are four broad categories:
In a winter rainfall continual cropping sequence of two or more crops the period between the harvest of one crop and sowing of the next crop represents the shortest fallow period. This is typically about 4 months. Since the short fallow commences after harvest, it has no impact at all on the previous winter-growing weed seed production. In wet summers, summer-growing weeds can be controlled but this has no direct in-crop weed management benefits in a winter cropping sequence, other than reduced nutrient tie-up and improved moisture accumulation.
In a winter rainfall pasture–crop sequence the period between weeding the pasture and sowing the first crop would be thought of as a long fallow and would have a duration of about 8 months. Because such fallows should commence well before weed seed maturity, they are a potent method of weed seedbank management.
In dry land summer crops can be grown, fallow periods exist between winter cereal harvest and the sowing of a summer crop (eg sorghum), a period of around 10 months.
Similarly, a fallow can exist between sorghum harvest through to cereal sowing in the following year, a period of around 14 months.
In low rainfall environments some farmers opt to ‘skip a year’ and call this a long fallow. Harvest would take place in year 1 and sowing would not occur again until year 3, a period of about 18 months. These long fallows embrace both a winter and summer growing season. The winter growing season presents a potent management option for winter growing weeds. Similarly, the summer season offers weed management options for summer-growing annual weeds.
In tropical and hot climate cropping systems it is also common to have consecutive winter-growing crops, depending on available subsoil moisture. As in category 1 above, the fallow period between these crops is about 6 months and has no impact on winter-growing weed species. Since it embraces a summer period, a short fallow will have an impact on summer growing annual weeds.
Winter rainfall and summer rainfall cropping systems are contrasted in Table A5.1. Note the length of the fallows in the summer rainfall system and the fact that winter fallows are a regular occurrence.
A fallow period can incorporate a number of strategies to reduce weed seedling and seed bank numbers. A range of non-selective control techniques can be used to practically prevent weed seed production. Options include grazing and cultivation or combinations of these. No in-crop or in-pasture weed treatment offers this level of weed control and reduced risk of evolving resistant weeds.
In a fallow phase it is easier to judge the efficacy of a weed control measure as there should be no surviving weeds. In-crop or in-pasture it is possible that surviving weeds can remain undetected by remaining hidden among desirable plants. By contrast, weeds that survive control measures are more obvious in fallows.
Whole-farm benefits
Soil moisture will be conserved. This is often cited as the number one advantage of fallowing. In lower and/or less reliable rainfall areas moisture conservation in-fallow is regarded as essential for reliable crop production.
Available nutrient levels will be optimised. A significant impact of weeds is to tie up available nutrients in their tissues. ‘Timely’ control versus ‘late’ control of fallow weeds has revealed a benefit of 40–50 kg of available N/ha, representing a significant saving in nitrogen fertiliser.
Fallow paddocks can provide fire protection for farms and livestock. Stubble-free fallows provide a safe refuge for stock during bushfires.
Control weeds of fallows when they are small. Small weeds are less likely to be stressed and are easier to control with cultivation, grazing or flaming in fallows. Small weeds also use less moisture and available nutrients.
Avoid over-reliance on cultivation. Cultivation increases the risk of erosion through loss of soil structure. If cultivation is used it should be for a range of reasons such as incorporating lime, mulch and other organic materials such as manure. Over-reliance on cultivation can also lead to a different range of weed problems, such as the spreading of perennial weeds.
Avoid cultivating wet soil. Cultivation of wet soil causes compaction and smearing.
Transplanting can also reduce the effectiveness of weed control.
Article: CulturalWeedControl201 (permalink) Date: 9 January 2013; 8:49:05 PM AEDT